Scenario for single WLAN to multiple VLANs

[sactoraymond]

Hi there,

I read from this forum some discussion about the WLC VLAN Select feature.

http://www.cisco.com/image/gif/paws/113465/vlan-select-dg-00.pdf

I see that you can use this feature to have multiple VLANS (interfaces) to map to the same WLAN (SSID).

What I try to learn is under what scenarios would people need to have mutliple vlan mapped to single SSID?

In my environment, I have 50+ AP int he campus on 20+ Cisco 4500 switches.  I have single WLAN and it is mapped to one subnet.  All wireless users would be on that subnets, whereas wired users are on 20+ subnets of their own.

Can someone help me to see under what scenarior (or requirement) that I would want to have multiple vlans mapped to single SSID?

Thanks.

[Scott Fella]

Many time the reason to use vlan select is to keep the subnet mask consistent. If you are use to having a /24 and want to keep a /24, them you can bundle a bunch of /24 subnet. Now there isn't anything wrong with having a loathe subnet because the WLC blocks broadcast traffic unless you allow it. In my install I have done both and both worked. Many of my large enterprise customers we have a /16 for guest and that works fine.

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[sactoraymond]

Hi Scott,

Where in the WLC config that would let you determine if "WLC blocks broadcast traffic"?

Thanks.

[Scott Fella]

Here is a screen shot.. Broadcast Forwarding set to Disabled

Thanks,

Scott

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[maldehne]

having a large number of users in single subnet is not the best in all designs, since you will have a large single broadcast domain which is a true disaster with dense networks. If you the company policy states that we need only one single ssid

for the whole employees within the company, it doesn't make sense to have them all on the same subnet.

A lot of options are available to overcome such issues :

for example, we might have AP groups feature , dynamic vlan assignment given that we have radius server in place, and vlan pooling.

It might not be feasible to have RADIUS server all the time, and AP groups might be kind of administrtive overhead as well as it might induce a lot of issues when aps fail over from controller to controller --> Vlan select is a good soultion considering the previously mentioned reasons.

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[sactoraymond]

Hi maldehne,

In your opinion, how many concurrent wireless users in a WLAN would you see it as "large enough" that you would want to break them down into individual subnets?

And a slightly different question.  In situation where there are multiple VLANs on the same SSID, how does the traffic from wireless user-A (in subnet A) route to the wireless user-B (in subnet B)?  I expect that the routing is done at the typical layer-3 router, not at the WLC.  Am I correct?

In my current setup, I have a firewall setup to control all the wireless traffic to the wired network.  If I am to break down the wireless network into mutlibple subnets, I assume I would need to truck the firewall interface for each subnet so I can continue govern the traffic.  Sounds right?

Thanks in advance.

[Scott Fella]

The WLC will not do any routing, it will just bridge the traffic.  I have not seen any issue with having a large subnet and other here have used large subnet in their production network.

Thanks,

Scott

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